1. Field of the Invention
The invention relates to inclination sensing instruments and more particularly to an instrument that utilizes variable capacitance as a function of inclination angle over all inclination angles.
2. Description of the Prior Art
Capacitive gravity sensors are well known and a particularly effective embodiment is discussed in U.S. Pat. No. 4,644,662 issued to Anderson et al on Feb. 24, 1987. FIGS. 1A and 1B illustrate the capacitive gravity sensor as taught in '662 patent. The gravity sensor includes a housing 10, a pair of capacitive plates including left plate 12 and right plate 14. The plates are at least partially immersed in a dielectric fluid 16.
In FIG. 1A the '662 inclination sensor is inclined at an angle .phi.. Details of a modification of the '662 sensor are shown in cross-sectional view at FIG. 1B where it can be seen that there are two left plates 12 separated by a dielectric board 18. In a preferred embodiment, this is a printed circuit board and has the electrically decoupled plates etched on both sides and is then covered with a TEFLON plastic or other insulative coating 15. The dielectric fluid 16 only partially fills the chamber formed by the housing 10 leaving a space thereabove which may be filled with a nitrogen gas 20. It should be noted that the two left plates are connected together and the two right plates are connected together, although the left and right pairs of plates are not connected together.
It will be seen that each plate with respect to the metallic housing 10 comprises a capacitor whose capacitance varies depending upon the amount of fluid in between the plates. The plates shown in FIG. 1A can also be seen from the front in FIG. 2A and from the rear in FIG. 2B. The fact that the two right plates are coupled together merely serves to increase the capacitance of the right plates and the same is true for the left plates.
The device shown in FIGS. 1A, 1B, 2A and 2B will provide accurate inclination information up to angles in the vicinity of 60.degree. to 70.degree.. As can be seen in FIG. 1A at angle .phi., the left plates are significantly more immersed in the dielectric fluid than are the right plates. Therefore, the capacitance between the left plates and the metal housing is increased and the capacitance between the right plates 14 and the metal housing 10 is decreased. This differential in capacitance can be measured and provides an electrical indication of the inclination of the sensor. This is discussed in the '662 patent and a suitable circuit for utilizing variable capacitance is discussed in U.S. Pat. No. 4,642,555 issued to Swartz et al on Feb. 10, 1987. The subject matter of U.S. Pat. Nos. 4,644,662 and 4,642,555 are herein incorporated by reference.
A problem with the above prior art inclination inclination angles of between .+-.60.degree.-70.degree.. This is apparent because it is at this point that one set of plates is completely immersed and the other set of plates is completely out of the dielectric fluid. Therefore, increases in angle do not result in a measurably increased capacitance on one side or a concurrent reduction in capacitance on the other side.